React.js, Facebook JS SDK and a costly little mistake

Last week I spent much more time than I ever wanted implementing Facebook login in React.js.

As it often turns out to be, one mistake cost me about 10 hours net of investigation, and if you don’t have time to read further, here’s what was wrong: I had added all my <script> tags inside <body> in the html, and then used <body> as the element to replace when rendering my main React component.

That was it. But what about that mistake is making me stop and write about it?

First: my app worked perfectly for a long time just as it was. Weeks. In the beginning of the project I was switching back and forth between two different build systems, and when I decided on one I must have copied the main React component and nothing pointed out an error.

Second: Facebook login kind of worked perfectly in localhost. The only strange thing I noticed was that, as I loaded the js sdk into the html, the most important sdk method, FB.init, was never found.

Since I am no React master at this point and Facebook provides no documentation on how to integrate FB login to React, even though they’re all made by FB, I had many possibilities to investigate. And lo and behold, by moving the FB.init call to the React component inside componentDidMount, everything about the login worked in localhost.

When it came to the staging environment, however, things got weird. My app would not get the login status on load, but on clicking the login button, the modal dialog would open. Sometimes the window would come up blank, sometimes it would prompt for login and then never close, and many other different outcomes.

Since most recent blog posts on Facebook login deal with difficulties with OAuth settings, my investigation went that way, and there it stayed for a long time. No warning or error message ever gave me the slightest clue as to what I was doing wrong.

Thus a mistake that arguably should have made my app break on the very first run ended up taking many hundreds of runs before the bug was caught.